Well or bore hole logging



Aug. 21, 1956 J. J. ARPs 2,760,079

WELL. OR BORE HOLE LOGGING' i" y arroz/Vf# United States Patent O WELL R BORE HOLE LOGGING Jan Jacob Arps, Tulsa, Okla.

Original application November 12, 1948, Serial No. 59,56. Divided and this application June 25, 1951, Serial No. 233,354

19 Claims. (Cl. .Z50-83.6)

Another object of the present invention resides in the provision of new and improved methods of and apparatus for obtaining indications of the nature of the earths structure or formation in the vicinity of the bore hole by conditioning the hole as by changing the radioactivity of the formations and Vthereafter observing the decay in radioactivity, as by logging the well at spaced 'time intervals.

A further object of the present invention is the provision of new and improved methods of and apparatus forA radiologically conditioning or contaminating the earths structure in the vicinity of 'a bore hole Vin order better to obtain indications of the nature of the earths structure or formation in the vicinity of the bore hole.

This application is a division of copending application Serial No. 59,656, tiled November 1'2, 1948, now

U. Patent No. 2,583,288.

It is known 'that certain characteristics of the earths lformation, such as permeability and porosity, inthe vicinity of a bore hole can be detennined by radioactive well logging, vas by running a Ygamma ray log of lthe bore hole. However, known methods require overriding of the natural radioactivity of the earths formations by the injection of mud or other carrier of high radioactivity into the bore hole before satisfactory indications can be obtained. This ,procedure may be dangerous because of the high radioactivity of the mud that must be injected into the bore hole. This diiiculty, according Yto one of the features of the present invention, lis leliminated vbecause it does not require the use of an overriding lhigh l radioactivity mud.

VIt s an object of the present invention to ,provide a new and improved method of well logging in which characteristics of the earths formation, such as permealiility and porosity, are logged 'by determining the changes in artificially induced contamination lof the 'formation.

`A further object of Vthe present invention is the provision of a new and improved method of Well logging which comprises running gamma ray well logs at prededetermined spaced time 'internals and 'comparing there- `stilts to determine the `decay in artificially induced radioactivity and thereby determine 'the permeability `and porosity of the 'subsurface structure.

A furt'le'r object "of the present `invention is to provide a new -'and improved method of Well logging in which "radioactivity mud is -trst 'forced into the bnrehle, the

2,760,079 Patented Aug. 21, 1956 r* ICC bore hole is then washed with nonradioactive mud, and two time spaced gamma ray well logs are run to determine the relative amounts of radioactivity and obtaining from them an indication of the formation permeability.

A further object of the present invention is to provide a new and improved method of well logging in which a relatively short lived radioactive tracer is released into the mud., as by generation and as an incident to the drilling and then circulated during the drilling process to contaminate the subsurface structure in the vicinity of the bore hole. Then two time spaced gamma ray logs are run to provide time spaced indications of the change in radioactivity from which the permeability and/ or porosity can be determined.

Another object of the invention is to provide a new and improved method of Well logging in which radioactive tracer is released into the drilling mud by continuous neutron bombardment.

A further object of the present invention is the Aprovision of new and improved methods for radiologically conditioning or .contaminating the earths structure in the vicinity of the bore .hole for the purpose of obtaining indications of the nature of the earths formation by bombarding .the formations with a neutron emitting substance.

A further object of the present .invention is to provide new and 'improved methods of and apparatus for obtaining indications of the nature of the earths structure or formation in the vicinity of .the lbore hole by injecting a neutron emitting substance .into 'the formations and thereafter logging 'the hole with Van instrument sensitive to neutrons.

A further 4object of the present invention resides in the vprovision of new and improved methods for obtaining indications of Athe nature of the earths structure lin .the vicinity of a bore hole by injecting into the bore hole -a substance that emits neutrons with a very short half life, waiting `for several of thesehalf lives., logging lfor gamma rays, waiting a period corresponding to several half lives of the expected artificially induced radioactivity and logging again.

Other objects and advantages of the present invention will become apparentfrom the ensuing .description of illustrative embodiments thereof, .in the course of which .reference is rhad to the accompanying drawings, .in which.:

Figs. 1 and 2 are illustrative graphs of time spaced gamma ray logs run :in accordance with the invention;

Fig. 3 is a ldiagrammatic representation of apparatus which lcan be used rin the practice of the invention; fand.

Figs. V4 and 5 are illustrations of contaminating apparatus made in .accordance with the invention.

In practicing the present invention, the well or lhere hole may :be contaminated by forcing radioactive mud into the bore hole. In the event the drilling is `carried-'on with `the use zof 'conventional nonradioactive mud, :a-

method which 'can lbe used is :substantially as follows: (l) replace the nonradioactive mud with radioactive lmud `and force the latter under pressure into the ;bore hole; (2) wash the borehole withnonradioactive mud to fclean kwashouts vin ywhich radioactive mud may have accumulated during =the preceding :step (this step is `not essential but is preferably used); vv(3) :run a iirst gamma .tray well log; (f4.) run a y'second gamma ray welllog after waiting for one or severalhalf lives of the radioactive material; and (5,) `compare fthe'two logs to determine -dissimilarities and thus obtain an :indication :of the formation permeability and porosity.

The gamma ray well logs may be standard radioactivity ilogs, see Well logging by radioactivity, fby tW.

G. -Greenan'd JR. Fearon, Geophysics, vol. V., No. 3.,

with a radiologic measuring instrument sensitive to gamma rays and a curve is plotted on an automatic recorder which is a usual part of the radioactivity logging apparatus.

The radioactive material can be introduced into the bore hole at any convenient time. This may be upon completion of the drilling operation or after an interesting zone has been drilled. When this occurs, the drilling operation is suspended and the above-described method followed.

The logs indicate the sum of two effects: (l) the natural radioactivity of the rocks, and (2) the radioactivity due to the infiltration of the radioactive fluid into the rock pores.

The results of an exemplary rst log are illustrated in Fig. 1 showing the passage through nine zones, A to l, inclusive, comprising sand, shale and limestone, as indicated.

After the passage of a predetermined length of time a second log is run. The results of this log are depicted in Fig. 2. The difference between the deflections along the X axis (horizontal) of the graphs of Figs. l and 2 indicates the effect due to the contamination of or the invasion of the radioactive lluid. The differences between the two graphs result from the decay of the articial contamination, which is present in the graph of Fig. 1, but which has disappeared from the graph of Fig. 2 because of decay with time.

Considering the graphs in detail, zone A is interpretable by standard radioactivity log interpretation as a sand and, since the deflections of the two logs are the same, it can be concluded that no fluid invasion took place and that the sand is nonpermeable. Similarly, zones B and H are interpreted as nonpermeable shale, zones C, E and I as nonpermeable sand, and zone F as nonpermeable limestone. Marked differences are apparent in zone D, there being a sharp peak to the right in Fig. l. This indicates an increase in radioactivity due to penetration of the radioactive mud into limestone. The zone would, therefore, be interpreted as a porous permeable streak in limestone. Another peak 12 appears inzone G and it is interpretable as a permeable streak in sand.

When the radioactive mud is injected into the bore hole, a certain amount penetrates into the formations in the proximity of the hole. The degree of penetration and the amount of penetration are functions of the permeability and porosity, respectively. As is well known, porosity is expressed by the percentage of the void space in a solid and in the ordinary rocks it is of the order of Permeability is a function of interconnection between the pore holes. Consequently, it is possible to have material which is porous but not permeable. Such material will have pore spaces which are not interconnected. As the mud injected into the bore hole invades the formation to a certain distance a certain amount of radioactive fluid will be accepted by the formation and this amount is a function of permeability and porosity. The radioactivity of the fluid accepted by the formation decays with time and it is this decay that is noted.

The radioactive material used preferably has a relatively short life. An extremely short life is impractical because of transportation time from the irradiating plant (atomic pile) to the well location. Cadmium 115 can be used. It has a gamma radiation of approximately .5 m. e. v. and half-life of 43 days. Indium 114, having a gamma radiation of .19 m. e. v. and a half-life of 48 days, can also be used. Using materials of this character requires a wait of four or ve months for their activity to decay substantially. Consequently, a relatively long period of time is necessary between the two logging operations. However, during this period, the well can be completely cased and cemented in conventional manner so that no danger of cave-in will exist.

The cementing and casing do not prevent the running of the second radioactivity log because the rays penetrate these materials. When a shorter waiting period is desired, radioactive isotopes of shorter half-life may be used, such as radio-iodine 131, which has gamma radiations of 0.367 and 0.08 m. e. v. and a half-life of 8 days and radio phosphorus 32, which has a beta radiation of 1.69 m. e. v. and a half-life of 14.3 days.

The mud can be rendered radioactive in suitable manner. For example, suitable radioactive materials can be added to the mud or the radioactive material can be generated and released into the mud stream above the surface of the ground. A suitable location for generating and releasing the tracer would be in the discharge pipe of a pump circulating the drilling fluid through the bore hole. The discharge pipe can be provided with a suitable chamber 110, as illustrated in Fig. 5. Within the chamber are mounted electrodes 112 and 114 connected to a suitable source of power 116. The electrodes are insulated from the chamber and made of radioactive material adapted electrolytically to dissolve in the mud stream by the current flow thereacross in the manner disclosed in my copending applications Serial Nos. 783,280 and 55,301, which matured as U. S. Patent Nos. 2,658,725 and 2,659,046 respectively, and as will be described hereinafter. Alternatively, apparatus like that shown in Fig. 4, which will be described hereinafter and which can be inserted into a chamber like pipe 110, can be used.

The present invention may be used advantageously with methods in which a contaminating tracer is released into the mud stream simultaneously with the drilling as an incident of a process of continuous radiologie logging, somewhat in the manner of the methods and apparatus disclosed in my above referred to copending applications. In thus simultaneously drilling and releasing the contaminating tracer, a radioactive material having a suitable half-life is used. Its half-life is such that it remains in the mud and is recirculated with it through the bore hole and thus injected into the earths formations. The two time spaced gamma ray logs are preferably run so that the rst is obtained immediately after the drilling operations have been completed and the second several months thereafter.

An apparatus that may be used is illustrated in Fig. 3. The apparatus here shown is illustrated drilling a well or bore hole 20. The drilling is done in accordance with conventional rotary drilling practice in which a circulating fluid, such as a suspension of clay solids in water and conventionally termed rotary mud is used to carry the drill cuttings out of the bore as the drilling proceeds. The upper portion of the bore hole is lined with a surface casing 22 extending, as usual, a comparatively short distance into the hole. A fluid outlet pipe 24 discharging into shale shaker 26 is connected to the upper end of the casing. The shaker separates the formation cuttings carried upward by the drill mud from the drilling mud itself. The latter passes from the shaker 26 to a conventional mud ditch 28 through a radioactivity measuring device 30 and a discharge nozzle 32.

Extending into the bore hole through the casing 22 is a conventional hollow drill string designated generally by the reference character 34 which is connected lat its lower end to a drill bit 36 having openings 38 adjacent its cutting edges. A Kelly joint 40 is connected to the upper end of the drill pipe and extends through a rotary table 42a mounted on the floor of a derrick. Rotation of the table 42a eects rotation of the Kelly joint 40 and drill stning 34 to Krotate the lbit 36. The upper end of the Kelly joint is connected to the usual rotary hose swivel 42 and the entire drill string assembly is suspended from a traveling block 44 adapted to be raised and lowered in the derrick in conventional manner by means including a cable 46.

The apparatus for circulating the drilling lluid through the well bore comprises the mud `ditch 28, a settling pit 48, a pump suction pit 50, and a mud pump 52 connected by 5.a suction pipe 54 .to the suction pit and by a discharge pipe :'56 to the drill pipe.

The volumetric ow of mud lflowing through the discharge pipe 56 may be measured by a fluid meter 58 of conventional type. The depth of the well may be measured by any conventional method. This is commonly done by adding together the measurements of lengths of all sections ofi-drill pipe lin the well and by suitable marking of the kelly to determine how much of -it lhas descended into 4the well. However, depth measurement can -bel made .by other suitable means, such, for example, as by a depth meter 6%) operatively connected by a measuring :line 62 to the traveling block. The meter end `of the line is :connected to a measuring pulley 464 `and the other end passes `over pulleys 66 to the traveling block.

The lower portion of the drill string 34 includes a drill collar 70, a portion of which is covered with an insulated sleeve '72 to the exterior of which are secu-red `a pia-ir of velectrodes 74 and 76, which in accordance with my previously referred to copending applications may vinclude cadmium 115 or indium 114 and are adapted to release a radioactive tracer material into the mud stream in quantities indicative of the conductivity or resistance of lthe earihs formation in the vicinity ofthe electrodes. The electrodes are, as fully disclosed in my copending -applications, supplied with voltage and are made of materials adapted electrolytically to generate and release radioactive `material incorporated in one or both of the electrodes.

'it is apparent from the foregoing that as the drilling progresses the drill collar with its radioactive releasing electrodes is lowered into the bore hole. Radioactive material is continually emitted into and circulates with the mud stream. ln accordance with the present invention, the radioactive material is not removed from the mud stream but is recirculated'through the bore hole with the result that the earths formation 'in the vicinity of the bore hole is injected with or invaded by the radioactive substance. The amount of penetration of the formations by yradio-active mud is dependent upon the permeability and porosity of the formations, as previously described. With the passage of time and continuous drilling, the walls of the bore hole are more or less permeated with the radioactive material. Then either upon completion -of the drilling or whenever desired, a pair of time spaced gamma ray logs as previously described are run in order to ascertain the decay in radioactivity in the time between the two logs. This, as heretofore .also indicated, provides information regarding the permeability of the subsurface formations.

If desired, the bore hole can be cleaned of the radioactive material by washing it with a neutral uid as described in connection with the first described embodiment of the invention. Alternatively, if the half-life of the radioactive material is sufficiently short so as to reduce the radioactivity substantially to zero when the second log is run, the mud can be left in the bore hole.

In the last-described embodiment of the invention it is desirable to include radioactive materials in the electrodes which have a relatively short life. The materials may be cadmium 115 or indium 114, iodine 131, or phospho-rus 32.

It may be noted, therefore, that the radioactive tracer may be injected into the bore hole during the process of continuous radiologie logging while drilling `and may actually 4constitute a by-product of that method. The infiltration into the formation takes place right at 'the bit and before serious mudding off has taken place, and is, therefore, superior to methods in which radioactive tracer is forced in `after a mud cake has formed. The use of short-lived tracers is particularly advantageous because it makes it possible readily to distinguish between the natural radioactive materials which are, by reason of their existence, of very long half-life, and injected tracer material, which has ya short life. 'Ihis difference in half-life provides the necessary means for differentiat- 6 ing -between the two types of radioactivity in the time spaced logs.

The mud and bore hole can also bec'ontaminated simultaneously with the drilling by .generating a radioactive tracer material in the drill collar by bombardment with neutrons. The neutrons may be derived from a `portable source carried in Ythe drill collar and comprising an intimate mixture of about 1 Curie-of radium salt with about 15 or 2O grams of finely ground beryllium. Other sources of neutrons may be used, such, for example, as any energetic alpha ray emitter mixed with beryllium powder. In Fig. V4 there is illustrated one arrangement that can be used. It includes an artificial radioactive source that is maintained activated continuously by `neutron bombardment. It comprises a central core of radium salt and beryllium powder. This core vis surrounded by paraffin 102 which serves to slow the neutrons to such an extent that a preponderance of them are so-called thermal or slow neutrons. The outer surface of the paraffin is covered with Ia coating 104 of substance to be activated and maintained activated by continuous neutron bombardment. This substance may `be phosphorus 32 with gamma. rays of 1.51 m. e. v. and a halflife of 12.4 hours; gold 198 having gamma rays of .44 m. e. v. and a half-life of 2.-'7 days; chromium 51 having gamma rays lof .32 m. e. v. and a half-life of 26.5 days.

ln accordance with a further feature of the invention the circulating or drilling mud can be contaminated with a neutron emitting substance. In this method, neutron emitting materials are mixed with the mud and circulated through the borehole. -For example, the neutron-emitting substance may be in the form of very small particles each consisting of a neutron emitting core covered with a paraffin coating. The neutron emitting core of eac-h particle may comprise radium salt or polonium salt intermixed with beryllium oxide. Then a pair of time spaced gamma ray logs are run, preferably after injection and washing of the bore hole as in connection with the first described method. -In this case the log corresponding to the contaminated condition, i. e., the .first -of the two logs, indicates a variable which is a function lof 'a .property of the formation. The vproperty that yenters 4into the log is not very definite `but can broadly be described as neutronie The neutronic lrocks can be recognized and thus detected by their ability to slow down neutrons, capture them, become nadioactive and emit gamma rays with a certain energy and a certain half-life 4which are characteristic of the rockin question. Correlatable logs can be obtained which will show the `n'eutronic rocks as a function of depth and can be compared from well to well giving valuable information regarding the geologic structure of the subsurface.

lInstead of injecting a radioactive tracer into the bore hole as in connection with the first of the methods described herein, a neutron 4emitting substance can be injected into the well. Time spaced logs ywith an instrument sensitive to neutrons are then run. This process is very similar to the first-described process, except that neutrons are used entirely.

A substance emitting neutrons with a very short halflife can be injected into the bore hole. A time equal to several of the half-lives of the neutrons is `permitted to elapse, after which a gamma ray .log is run. After a second period corresponding to several 'half-lives of the expected artificially induced radioactivity, a second gamma ray log is run. The difference between the last two logs is indicative of the radioactivity artificially nduced by the neutrons.

While different embodiments of the invention have been described, it should be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. A method of logging an earth borehole which comprises: injecting a uid carrying radioactive tracer material into the borehole under sufficient pressure to force a portion thereof into the penetrated porous formations surrounding said borehole; running a first radioactivity Well log in said borehole; Waiting a time interval thereafter sufficient to permit a measurable decay in the radioactivity of the tracer material therein; and then running a second radioactivity well log in said borehole, to obtain by comparison of such logs information regarding characteristics of the said formations.

2. A method of logging an earth borehole which comprises: injecting a fluid carrying radioactive tracer material into the borehole under sufficient pressure to force a portion thereof into the penetrated porous formations surrounding said borehole; running a first gamma ray well log in said borehole; waiting a time interval thereafter sufiicient to permit a measurable decay in the gamma ray radioactivity of the tracer material therein; and then running a second gamma ray well log in said borehole, to obtain by comparison of such logs information regarding characteristics of the said formations.

3. A method of logging an earth borehole which comprises: injecting a liuid carrying a time decaying detectable tracer material into the borehole under sufficient pressure to force a portion thereof into the penetrated porous formations surrounding the said borehole; running a first log in said borehole to determine the apparent relative distribution of the said tracer along said formations in said borehole; Waiting a time interval thereafter sufficient to permit a measurable decay in said tracer material; then running a second log in said borehole to determine the apparent relative vdistribution of the said tracer along said formations in said borehole, to obtain by comparison of such logs information regarding characteristics of the said formations.

4. A method of logging an earth borehole which comprises: injecting a fluid carrying radioactive tracer material into the borehole under sufficient pressure to force a portion thereof into the penetrated porous formations surrounding the said borehole; running radioactivity well logs in said borehole at time intervals therebetween sufficient to permit measurable decay in the radioactivity of the tracer material therein during said time intervals, to obtain by comparison of such logs information regarding characteristics of the said formations.

5. A method of logging an earth borehole which comprises: circulating during the drilling of said borehole a drilling fluid carrying a radioactive tracer material and under sufficient pressure to force a portion thereof into the penetrated porous formations being drilled; running a first radioactivity well log in said borehole; waiting a time interval thereafter sufficient to permit a measurable decay in the radioactivity of the tracer material therein; and then running a second radioactivity well log, to obtain by comparison of such logs information regarding characteristics of said formations.

6. A method of logging an earth borehole which comprises: circulating during the drilling of said borehole a drilling uid carrying a radioactive tracer material and under sufficient pressure to force a portion thereof into the porous formations being drilled; washing the borehole with nonradioactive drilling fluid; running a first radioactivity Well log in said borehole; Waiting a time interval thereafter sufficient to permit a measurable decay in the radioactivity of the tracer material therein; and then running a second radioactivity well log, to obtain by comparison of such logs information regarding characteristics of the said formations.

7. A method of logging an earth borehole which cornprises: circulating fluid through the borehole during dr-illing; adding to said uid in the borehole during drilling a time decaying detectable tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; and thereafter substantially short of complete decay of the contaminating tracer material running a pair of tracer logs spaced apart in time relative to one another suicient to permit during such time a measurable decay in the said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

8. A method of logging an earth borehole which comprises: circulating tiuid through the borehole during drilling; adding to said uid in the borehole during drilling a time decaying detectable tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; washing said borehole with fluid free of said tracer material; and thereafter substantially short of complete decay of the contaminating tracer material running a pair of tracer logs spaced apart in time sufficient to permit during such time a measurable decay in said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

9. A method of logging an earth borehole which comprises: circulating uid through the borehole during drilling; generating and releasing into said fluid adjacent the top of the borehole during drilling a relatively short-lived radioactive tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; and thereafter substantially short of complete decay of the thus-generated contaminating tracer materral, running a pair of tracer logs spaced apart in time relative to one another sufficient to permit during such time a measurable decay in the said generated tracer material, to obtain by comparison of such logs information regardin g characteristics of the said formations.

l0. A method of logging an earth borehole which cornprises: circulating fluid through the borehole during drilling; generating and releasing into said uid adjacent the Alower end of the borehole during drilling a relatively short-lived radioactive tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; and thereafter substantially short of complete decay of the thus generated contaminating tracer material running a pair of tracer logs spaced apart in time relative to one another suicient to permit during such time a measurable decay in the said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

1l. A method of logging an earth borehole which comprises: circulating fluid through the borehole during drilling; generating and releasing into said iiuid adjacent the top of the borehole during drilling a relatively short-lived radioactive tracer :material capable of contaminating the earth formations in the Vicinity of the borehole being drilled, washing said borehole with lluid free of said tracer material; and thereafter substantially short of complete decay of the thus generated contaminating tracer material running a pair of tracer `logs spaced in time relative to one another sufficient to permit during such time a measurable decay in the said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

12. A method of logging on earth borehole which comprises: circulating fluid through the borehole during drilling, generating and releasing into said fluid adjacent the lower end of the borehole during drilling a relatively short-lived radioactive tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; 'washing the borehole with iiuid free of said generated tracer material; and thereafter rsubstantially short of complete decay of the thus generated contaiminating tracer material running a pair of tracer logs spaced apart in time relative to one another sufiicient to permit during such time a measurable decay in the said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

13. A method of logging an earth borehole which comprises: circulating iiuid through the borehole during drilling, generating by neutron bombardment and releasing into said fluid adjacent the top of the borehole duringv drilling a relatively short-lived radioactive tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; and thereafter substantially short of complete decay of the tracer logs spaced apart in time relative to one another sufficient to permit during such time a measurable decay in the said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

14. A method of logging an earth borehole which comprises: circulating uid through the borehole during drilling; generating by neutron bombardment and releasing into said uid adjacent the lower end of the borehole during drilling a relatively short-lived radioactive tracer material capable of contaminating the earth formations in the vicinity of the borehole being drilled; and thereafter substantially short of complete decay of the thus generated contaminated tracer material running a pair of tracer logs spaced apart in time relative to one another su'cient to permit during such time a measurable decay in the said tracer material, to obtain by comparison of such logs information regarding characteristics of the said formations.

15. Apparatus for logging a bore hole in the earth to determine a characteristic of the geological formation surrounding the hole including means for circulating a carrier fluid through the hole, and radioactive tracer generating means located at the surface of the earth in the path of circulation of the fluid for adding a radioactive tracer to said fluid.

16. Apparatus for logging a bore hole in the earth to determine a characteristic of the geological formation surrounding the hole including means for circulating a carrier uid through the hole, and radioactive tracer generating means located at the surface of the earth and including spaced electrodes, at least one of which includes radioactive material, in the path of circulation of the fluid for adding a radioactive tracer to said fluid.

17. Apparatus for logging a bore hole in the earth to determine a characteristic of the geological formation surrounding the hole including means for circulating a carrier lluid through the hole, and radioactive tracer generating means located at the surface of the earth in the path of the circulating fluid for adding a radioactive tracer to said fluid, lsaid means including a core of neutron emitting material, paratlin surrounding the core, and a coating over said paraffin adapted to be activated by neutron bombardment.

18. Apparatus for logging a bore hole in the earth to determine a characteristic of the geological formation surrounding the hole including means for circulating a carrier fluid through the hole, and radioactive tracer generating means movable into the hole for adding a radioactive tracer to said fluid, said means including a core of neutron emitting material, paraffin surrounding the core, and a coating on said paraflin adapted to be activated by neutron bombardment.

19. Apparatus for logging a borehole in the earth to determine a characteristic of the geological formations surrounding the hole including means for circulating a carrier uid through the hole, and radioactive tracer generating means located in the borehole in the path of circulation of the fluid for adding a radioactive tracer to said uid.

References Cited in the file of this patent UNITED STATES PATENTS 2,308,176 Howell Jan. l2, 1943 2,352,993 Albertson July 4, 1944 2,385,378 Piety Sept. 25, 1945 2,390,931 Fearon Dec. 11, 1945 2,480,674 Russell Aug. 30, 1949 

1. A METHOD OF LOGGING AN EARTH BOREHOLE WHICH COMPRISES: INJECTING A FLUID CARRYING RADIOACTIVE TRACER MATERIAL INTO THE BOREHOLE UNDER SUFFICIENT PRESSURE TO FORCE A PORTION THEREOF INTO THE PENETRATED POROUS FORMATIONS SURROUNDING SAID BOREHOLD; RUNNING A FIRST RADIOACTIVITY WELL LOG IN SAID BOREHOLE; WAITING A TIME INTERVAL THEREAFTER SUFFICIENT TO PERMIT A MEASURABLE DECAY IN THE RADIOACTIVITY OF THE TRACER MATERIAL THEREIN; AND THEN RUNNING A SECOND RADIOACTIVITY WELL LOG IN SAID BOREHOLE, TO OBTAIN BY COMPARISON OF SUCH LOGS INFORMATION REGARDING CHARACTERISTICS OF THE SAID FORMATIONS. 